Process for crystalline salts of L or (S)-3-(3,4-dihydroxyphenyl)-2-methylalanine esters

ABSTRACT

Crystalline salts of (R,S)-pivaloyloxyethyl esters of (S)-3-(3,4-dihydroxyphenyl)-2-methylalanine and processes for their preparation are disclosed.

The present invention is concerned with certain salts of (R,S)pivaloyloxyethyl esters of L or(S)-3-(3,4-dihydroxyphenyl)-2-methylalanine in a crystalline form and toprocesses for their preparation.

BACKGROUND OF THE INVENTION

L or (S)-3-(3,4-dihydroxyphenyl)-2-methylalanine, more commonly known asmethyldopa, is a well-recognized antihypertensive agent which isgenerally administered orally in the form of capsules, pellets, oremulsions. Certain esters of methyldopa have been found to be of greaterpotency, Saari et al., J. Med. Chem. 21, 746 (1978). One of the estersfound to be of higher potency is the 1-pivaloyloxyethyl ester of(S)-3-(3,4-dihydroxyphenyl)-2-methylalanine. Preparation andpharmaceutical applications of these pivaloyloxyethyl esters are alsodescribed in U.S. Pat. Nos. 3,983,138; 3,988,341; 4,016,288 and4,051,169. The latter patent also teaches that both isomers arising as aresult of the chiral center in the substituted alkyl portion of theester are active pharmacologically. Subsequent work has supported theefficaciousness of the pivaloyloxyethyl ester and the desirability ofemploying this highly effective ester product in special means foradministration such as in controlled delivery systems. Controlleddelivery systems include not only a delayed delivery method such asenteric coated tablets but also and particularly, delivery in devicessuch as an osmotic pump for controlled and continuous release of drugs.A representative device is described in U.S. Pat. No. 4,265,874. Forefficient and effective use in such device, it is highly desirable thatthe drug be in a pure crystalline form, and further exhibit appropriatesolubility properties for controlled release in an aqueous environment.However, the pivaloyloxyethyl ester tends to be unstable even whenobtained in a crystalline form. Thus, it is desirable to provide amethod for obtaining the pivaloyloxyethyl esters of methyldopa not onlyin a crystalline form, but also in a stable form. Further, it isdesirable to have the ester in a form having solubility propertiesadaptable for controlled release in an aqueous environment. Moreover,since as taught in U.S. Pat. No. 4,051,169, both isomers of the estersare pharmacologically active, it is desirable to provide for a method inwhich both isomers may be obtained in crystalline form without unduedifficulty.

STATEMENT OF THE INVENTION

According to the present invention there has been discovered certaincrystalline salts of the diastereomeric (R,S) mixture ofpivaloyloxyethyl esters of (S)-3-(3,4-dihydroxyphenyl)-2-methylalaninewhich are stable to heat and compression, encountered during formulationand/or storage, and which have solubility properties suitable for use inosmotic delivery devices for controlled and continuous release of drugs.There has further been discovered a process for directly obtainingcrystalline salts of both diastereomeric isomers in a single operation,i.e., without need for the usual steps of first causing crystalformation of one isomer, removing the crystals from the mixture andthereafter causing the crystal formation of the second isomer. A furtheraspect of the process is the preparation of these crystalline (R,S)salts from the reaction mixture obtained in the synthesis of (R,S)esters. Another aspect is a process for simultaneous preparation of acertain group of (R,S) salts. Other aspects include pharmaceuticalcompositions comprising said crystalline ester salts suitable for use inthe control of hypertension.

DETAILED DESCRIPTION OF THE INVENTION

The crystalline (R,S) salts of the present invention may be representedby the formula: ##STR1## wherein HX represents the acid forming the saltwith the ester base; ROH represents the solvent of the solvate and n isa number of from 1/2 to 3. Although the salt is written above andhereinafter in the specification as an acid addition salt, it is to beunderstood that the salt formation takes place with the proton attachingat the amino nitrogen and may be written as follows: ##STR2## In theforegoing formula, X represents the residual group remaining afterremoval of a proton from the acid, HX, and R in ROH represents hydrogenor lower alkyl depending on whether the solvent forming the solvate isalcohol or water; and n is as hereinbefore defined.

The acids which are to be employed to form the desired salts areoxygenated acids. By "oxygenated acids" is meant an inorganic or organicacid containing an oxygen atom to which a proton is attached. Preferredinorganic acids include phosphoric and sulfuric acids. Preferred organicacids include tartaric acid, maleic acid, malonic acid, and the like.Some of the organic acids have chiral centers; when such acids areemployed it is desirable to employ a single enantiomorph. More than oneacid may be employed with the same ester to form salts of mixed acids.Thus, by using phosphoric acid and tartaric acid, a phosphate/tartratesalt mixture may be obtained. In such cases, HX in Formula I representstwo acids and also may be designated HX₁.HX₂ where X₁ and X₂ aredifferent anionic groups.

The solvate forming solvents contemplated by the present invention arenon-acidic, hydroxylated solvents, particularly alkanols and water.Thus, ROH includes water, ethanol, pentanol-1, pentanol-2, isopropanol,propanol, methanol and the like. Preferred solvates are ethanolate andhydrate from ethanol and water, respectively. These solvates are alsopreferred for use in osmotic delivery devices.

Novel crystalline salts of the present invention named as acid additionsalts and in which (R,S)-1-pivaloyloxyethyl(S)-3-(3,4-dihydroxyphenyl)-2-methylalaninate is designated (R,S)-POEester methyldopa include crystalline (R,S)-POE ester methyldopa.H₃PO₄.H₂ O; (R,S)-POE ester methyldopa.H₃ PO₄.C₂ H₅ OH; (R,S)-POE estermethyldopa.(+)tartaric acid.H₂ O; (R,S)-POE ester methyldopa.H₃ PO₄.CH₃OH; (R,S)-POE ester methyldopa.H₂ SO₄.C₂ H₅ OH; (R,S)-POE estermethyldopa. H₂ SO₄.H₂ O; (R,S)-POE ester methyldopa. (-)tartaric acid.H₂O; (R,S)-POE ester methyldopa.DL-tartaric acid.H₂ O; (R,S)-POE estermethyldopa.meso-tartaric acid.H₂ O; (R,S)-POE ester methyldopa.maleicacid.H₂ O.

Especially preferred salts for use in pharmaceutical compositions are(R,S)-1-pivaloyloxyethyl (S)-3-(3,4-dihydroxyphenyl)-2-methylalaninate(+)-tartaric acid/phosphoric acid (3/1) monohydrate and(R,S)-1-pivaloyloxyethyl (S)-3-(3,4-dihydroxyphenyl)-2-methylalaninatephosphoric acid monoethanolate.

The salts may have an apparent fractional amount of water of hydrationor solvent of solvation attributable either to mixtures of solvated andunsolvated salts or the solute "bridging" two salt molecules. The extentof solvation is preferably in the range of 1/2 to 1; i.e., n is from 1/2to 1.

The process may be applicable to formation of salts from other acids butfor the intended purposes, the foregoing novel crystalline salt mixturesare a preferred embodiment of the present invention.

As can be seen in Formula I, there are two points in the formula whichmay be designated as chiral centers. One point is on the amino acidportion, specifically the carbon atom to which the amino nitrogen isattached and the other is in the hydroxy compound portion of the ester,specifically the carbon atom attached to the oxygen of the ester groupand designated with an asterisk (*). Since the pharmacologically activeamino acid, methyldopa, is always the L or (S) form of3-(3,4-dihydroxyphenyl)-2-methylalanine, only the L or (S) form of theacid is employed in the preparation of the ester, and the chiral centeron the amino acid portion of the above formula is always (S) and doesnot contribute to the formation of isomeric mixtures during saltformation. Since 1-chloroethyl pivalate used in the preparation of1-pivaloyloxyethyl ester (POE ester) of methyldopa has a chiral center,the resulting esters are a mixture of (R) and (S) diastereomers, and thesalts resulting from the esters would also be a mixture ofdiastereomers. As has been previously established, both isomers arepharmacologically active, thus, salts of both isomers are desired.However, as generally known, salts of diastereomers not only separate ina stepwise manner but almost always require isolation and removal of thecrystals of one isomer before separation of the second isomer can bemade to occur. Moreover, separation of the second isomer usuallyrequires extensive manipulative procedures; however, if such proceduresare not taken then about one-half of the ester is unutilized and wasted,a commercially unfeasible operation.

Thus, it was wholly unexpected that according to the process of thepresent invention a crystalline mixture of salts corresponding to themixture of (R) and (S) POE esters of(S)-3-(3,4-dihydroxyphenyl)-2-methylalanine may be produced without theneed for first isolating the salt of one of the ester isomers, therebymaking it possible to avoid, on the one hand, an onerous and expensiveprocedure which would be necessitated by first isolating one isomer andusing extensive manipulative procedures to recover the second isomer,and on the other hand, a wasteful procedure of forming the salt of asingle ester isomer and discarding the mother liquor. A further aspectof the present invention is a process not only of preparing acrystalline mixture of salts without the need for first isolating oneisomer but also for preparing a crystalline mixture of salts of theisomers from the reaction mixture formed in the ester synthesis withoutthe need for recovering the esters from the reaction medium as a solid.

In the process of the present invention, the crystalline salts areprepared by reacting an isomeric mixture of esters, namely(R,S)-1-pivaloyloxyethyl (S)-3-(3,4-dihydroxyphenyl)-2-methylalaninate(hereinafter also designated R,S-POE ester base) and a salt forming acidin an organic solvent. The organic solvent may be the solvate formingsolvent as hereinbefore defined or other common inert organic solvent,water-immiscible or miscible. For consistent good results, the reactionpreferably is carried out in a substantially water-immiscible organicsolvent with inclusion of a very small amount of a hydroxylated solvent.Thus, the preferred process contemplated for preparing a diastereomericsalt mixture of 1-pivaloyloxyethyl ester of

(S)-3-(3,4-dihydroxyphenyl)-2-methylalanine comprises

(a) forming a solution of (R,S)-1-pivaloyloxyethyl(S)-3-(3,4-dihydroxyphenyl)-2-methylalaninate in a non-polarsubstantially water-immiscible organic solvent,

(b) adding thereto a solvate forming hydroxylated solvent and a saltforming oxygenated acid, and

(c) intimately contacting the mixture preferably under an atmosphere ofnitrogen for time sufficient to cause crystallization of the salts of Rand S isomers.

The reaction may be carried out entirely at ambient temperature.However, generally after initial crystal formation subsides, it isdesirable to cool the reaction mixture and maintain the mixture in thecooled state to complete the reaction with the formation of the salts ofR,S-POE ester base in good yields. Generally yields of 80 percent orgreater may be obtained.

By the expression "ester base" is meant POE ester as hereinbeforedefined but used in the context of conversion to a salt or vice-versa.The reactant R,S-POE ester base is preferably pure or purified esterbase. The ester base may be obtained by the reaction of(R,S)-1-chloroethyl pivalate with(S)-3-(3,4-dihydroxyphenyl)-2-methylalanine as described in theaforecited Saari et al. paper and thereafter purified or may be preparedmore directly as a crystalline ester as described in copendingapplication Ser. No. 353,697, filed Mar. 1, 1982 in the name of JohnBudavari and also described hereinafter in connection with the procedurefor preparation of the salts from a reaction mixture obtained in thepreparation of the esters. For certain salts, it is important that theester base be of a purified crystalline material. For others, while notas important that crystalline ester base be employed, it is advantageousto subject the ester base to purification procedures which is describedhereinafter.

When the starting ester base is of at least 95 percent purity and thesalt is to be an alkanolate the reaction may be carried out in theabsence of a water-immiscible organic solvent with the alkanol servingas a dispersion medium.

When the reaction is carried out according to the preferred process in asubstantially water-immiscible organic solvent, the salt forming acidand the solvate solvent may be added successively, simultaneously or inthe form of a solution of the acid in the solvate solvent. It ispreferably added in the form of a solution of the acid in a solvatesolvent. Moreover, it is preferably added to the reaction medium in adropwise manner.

Suitable solvents for the substantially water-immiscible reaction mediumin the salt preparation include toluene and other alkylated aromatichydrocarbons, halogenated aliphatic and aromatic hydrocarbons, esters,such as ethyl acetate, isopropyl acetate and the like, and ethers, suchas diethyl ether, diisopropyl ether and the like. Preferred solvents areesters such as ethyl acetate and alkylated aromatic hydrocarbons such astoluene.

The actual amount of solvent depends on the solvent employed and thesolubility of the POE ester base in the solvent. It is highly desirablethat the amount be such that the ester base not exceed about 10 percentby weight. Generally an amount to provide a 2 to 5 weight percentsolution of the ester base in solvent would be considered satisfactory.

When the ester base is in a crude or unpurified state, preliminarypurification steps are employed prior to the preparation of saltcrystals utilizing the ultimate reaction solvent. The purification maybe carried out by thoroughly mixing the crude ester base with thesolvent to be employed in the salt preparation to dissolve the esterbase therein, and then separating the insoluble material (predominantlymethyldopa) by suitable manipulative procedures such as filtering,washing, and the like. Thereafter, the ester base solution is employedin the foregoing crystal salt formation process, after first takingappropriate steps, if necessary, to bring the solution into anappropriate concentration range for reaction.

The reaction is preferably carried out in an atmosphere of nitrogen orother inert gas.

The temperature for crystallization varies from ambient temperature to-25° C. Ambient temperature is meant a temperature in the range fromabout 15° C. to about 30° C. The preparation of salts as alkanolates,particularly from mineral acids such as phosphoric acid, may be carriedout entirely at ambient temperature. However, it is generally preferableto keep the temperature below at least about 25° C. so some cooling asnecessary is usually employed. After initial separation of crystalscooling is preferably employed to temperatures near 5° C. and below toas low as about -25° C. to induce further separation of crystals.Crystallization occurs without removal of the crystals initially formed.

Although not essential, the reaction mixture usually is seeded with asalt prior to each crystallization. While any conventional means forinducing crystallization may be employed, crystallization is greatlyfacilitated by seeding. Suitable seeds are not only previously preparedidentical salt solvate but also any solvate of the correspondingcrystalline salt.

After completion of the reaction, the R,S salt mixture may be recoveredby filtration and purified, if desired, by washing with solvent anddrying under reduced pressure.

During the course of the reaction, the formation of the crystallinesalts may be followed by thin layer chromatography (TLC) by sampling thecrystals, washing with solvent, and placing on silica gel plates,developing with solvent mixture, preferably 60:25:10:5-C₂ H₅ OCOCH₃:n-C₄ H₉ OH:HCOOH:H₂ O and visualizing with I₂. Alternatively, samplesmay be taken and checked by optical rotation.

The process is also adaptable to the preparation of salts of more thanone acid. For the preparation of salts of multiple acids, a crystallineR,S mixture of salts from a single acid is first prepared as abovedescribed. Thereafter, the second acid and the solvate forming solvent,preferably as a solution of acid in solvent, are added, usually directlyto the crystalline mixture of salts from the first acid, and theresulting mixture stirred with cooling, generally below 0° C. to obtaincrystals of the second salt to obtain an R,S mixture of salts derivedfrom two acids, HX₁.HX₂. After substantial completion of thecrystallization, the crystalline product of mixed salts are recovered byfiltration, washed with solvent and dried.

In the preparation of the mixed salts, temperature control at the timeof formation of the second salt is particularly critical. It should notbe permitted to exceed about 25° C. and is generally carried out below0° C. or lower and as low as -25° C.

An important aspect of the process of the present invention is thepreparation of the salts of the R,S-POE esters directly from the crudereaction mixture obtained in the synthesis of (R,S)-1-pivaloyloxyethyl(S)-3-(3,4-dihydroxyphenyl)-2-methylalaninate by alkylation of(S)-3-(3,4-dihydroxyphenyl)-2-methylalanine with 1-chloroethyl pivalate.In the preparation of the salts from the crude alkylation reactionmixture, the reaction solvent from the alkylation step is first replacedwith a water-immiscible organic solvent, the resulting solution freed ofacid impurities and then used to prepare the crystalline salt mixture ofR,S-POE ester in the manner previously described for the preparation ofcrystalline salt mixture starting from isolated R,S-POE ester ofmethyldopa.

The POE ester base containing reaction mixture is that which may beobtained by mixing together and reacting substantially equimolarproportions of methyldopa and 1-chloroethyl pivalate in an aproticsolvent in the presence of a molecular sieve, preferably in anatmosphere of nitrogen, and preferably with moderate heating over aperiod of time sufficient to complete the reaction with the formation ofthe desired ester as described in copending application of John Budavaripreviously referred to.

In this process, substantially equimolar amounts of methyldopa and1-chloroethylpivalate, and a molecular sieve, in an amount preferably inthe range of 30 to 40 grams per 100 grams of methyldopa, are placed inan aprotic solvent, said solvent being employed in an amount of fromabout 1 to 10 milliliters for each gram of methyldopa, and the mixturestirred at temperatures in the range of from about 25° C. to about 150°C., preferably about 70° to 100° C. for time sufficient to complete thereaction with the formation of the POE esters of methyldopa.

Suitable molecular sieves are of crystalline metal aluminosilicates inpowder, pellets and beads. They are readily available commercially andare referred to in pore size (angstrom) designations such as3A,4A,5A,13X and the like.

The "aprotic solvent" in which the reaction is carried out is a materialwhich is liquid under reaction conditions, which at least partiallydissolves the reactants and which does not readily yield or accept aproton. It is preferably miscible with water. Solvents suitable forcarrying out the reaction include dimethyl sulfoxide, formamide,tetramethylurea, tetraethylurea, cyclic ureas, N,N-dialkylacetamides,hexamethylphosphoramide, tetrahydrofuran, N-alkyl-pyrrolidinones,acetonitrile, 1,3-dimethyl-2-imidazolidinone,1,3-dimethyl-3,4,5,6-tetrahydro-2(H)-pyrimidinone, and the like andmixtures thereof.

The reaction mixture obtained as above described, containing the R and SPOE esters of methyldopa (ester bases), and also containing unreactedstarting materials, by-products, molecular sieve and reaction solvent,is first diluted with a water-immiscible organic solvent and then washedwith an aqueous base to remove undesired acidic material. The aproticsolvent is also removed in the washing process, having been replaced bythe water-immiscible organic solvent. To the washed solution containingthe ester bases, there is added a salt forming acid corresponding to thesalt desired and a hydroxylated solvent, and the mixture stirred toobtain crystalline solvated salt of (R,S)-1-pivaloyloxyethyl ester ofmethyldopa in the manner previously described for the process startingfrom an isolated isomeric ester base mixture. The product salt may bepurified by washing with solvent and drying under reduced pressure.

The base used above for washing the ester bearing solution may be anywater-soluble, organic solvent-insoluble base, and is preferably aninorganic base. A preferred base is sodium bicarbonate, although sodiumcarbonate, sodium chloride, calcium chloride and the like also may beemployed.

Alkanolate salts, particularly those derived from inorganic acids, maybe obtained in a process whereby purified POE ester base mixture andappropriate acid are admixed in excess alkanol in the presence of aminor amount of water. By "excess" is meant more than the molar amountwhich would be necessary to form an alkanolate. The amount should bethat sufficient to permit smooth mechanical mixing of a slurry ofreactant ester and product salt or that sufficient to form a solutionwhen a minor amount of water is added. In this process, the desired R,Smixture of salts are substantially instantaneously formed as alkanolatesin substantially crystalline form. Sometimes some amorphous material maycoprecipitate but these are readily convertible to crystalline salts.However, the process applicable to all solvates is one in which theester base and acid are reacted in a water-immiscible organic solvent aspreviously described.

The foregoing procedures and modifications described provide a simpleand economical process for obtaining crystalline salts which can be ameans for supplying 1-pivaloyloxyethyl(S)-3-(3,4-dihydroxyphenyl)-2-methylalaninate in a form suitable forspecial drug delivery systems such as, for example, an osmotic pump.Thus, the salts so prepared show good solubility characteristics andosmotic pressure properties in aqueous systems which can be correlatedwith compositions suitable for use in osmotic pumps. The solubilityproperties are outstanding and highly unexpected since the saltsgenerally found to be useful for enhancing solubility such ashydrochloride solvates do not exhibit these outstanding properties. Thesolvate salts derived from oxygenated acids and POE esters of methyldopahave been found to increase the solubility of methyldopa in aqueousmedia to the extent of 15 times and more. Thus, methyldopa has asolubility of 10 grams per liter, whereas the solvated phosphate saltmixture of (R,S)-POE ester of methyldopa has a solubility in the rangeof 57 to 197 grams per liter and the solvated tartaric acid salt mixturehas a solubility in the range of 156 to 383 grams per liter. (Thesolubility of the salts has been found to be system dependent, i.e.,dependent on the amount of salt present and the relative amount of the(R) and (S) salts.) In addition, the POE ester salts also have beenfound to have advantageous properties over the POE ester bases whichtend to be unstable when subjected to an aqueous environment or tomechanical stress such as grinding.

Pharmaceutical compositions comprising the salts of the presentinvention constitute an aspect of the present invention. However, sincethe usefulness of the novel crystalline salts of the present inventionis the provision of a means for rendering 1-pivaloyloxyethyl(S)-3-(3,4-dihydroxyphenyl)-2-methylalaninate adaptable to specialmethods of administration, as well as providing a superior form of thedrug, and not to impart a new pharmacological activity, the amounts tobe employed in pharmaceutical compositions are readily ascertainable bythe skilled artisan from the literature including the previously citedpublication in the Journal of Medicinal Chemistry. For use in an osmoticdelivery device the salts may be formulated employing materials suitablefor methyldopa POE ester salts in their pharmaceutical application butprepared in a manner similar to that described in U.S. Pat. No.4,265,874 for a different drug and use.

While it is necessary to provide a product with certain purity andsolubility properties for use with delivery devices, the products ofsaid purity and solubility are also useful in conventional modes ofadministration and also may be employed in more conventionalpharmaceutical compositions for reducing hypertension.

For conventional methods of administration, the salts of the presentinvention may be contained in compositions preferably administered inunit dosage form such as tablets, pills, capsules, powders, granules,sterile parenteral solutions or suspensions, oral solutions orsuspensions and the like. For preparing solid compositions such astablets, the principal active ingredient is mixed with conventionaltableting ingredients such as corn starch, lactose, sucrose, sorbitol,talc, stearic acid, magnesium stearate, dicalcium phosphate, gums andfunctionally similar materials as pharmaceutical diluents or carriers.The tablets or pills of the novel compositions can be laminated orotherwise compounded to provide a dosage form providing the advantage ofprolonged or delayed action or predetermined successive action of theenclosed medication. For example, the tablet or pill can comprise aninner dosage and an outer dosage component, the latter being in the formof an envelope over the former. The two components can be separated byan enteric layer which serves to resist disintegration in the stomachand permits the inner component to pass intact into the duodenum or tobe delayed in release. A variety of materials can be used for suchenteric layers or coatings, such materials including a number ofpolymeric acids or mixtures of polymeric acids with such materials asshellac, shellac and cetyl alcohol, cellulose acetate and the like. Aparticularly advantageous enteric coating comprises a styrene maleicacid copolymer together with known materials contributing to the entericproperties of the coating. The compounds are also useful whenadministered in the form of suppositories or with a penetrant such asdimethyl sulfoxide.

The liquid forms in which the novel composition of the present inventionmay be incorporated for administration include suitably flavoredemulsions with edible oils, such as cottonseed oil, sesame oil, coconutoil, peanut and the like, as well as elixirs and similar pharmaceuticalvehicles. Suitable dispersing or suspending agents for aqueoussuspensions include synthetic and natural gums, such as tragacanth,acacia, alginate, dextran, sodium carboxymethylcellulose,methylcellulose, polyvinylpyrrolidone, gelatin and the like. Sterilesuspensions or solutions are required for parenteral use. Isotonicpreparations containing suitable preservatives are also highly desirablefor injection use.

The term single dosage form as used in the specification refers tophysically discrete units suitable as unitary dosage for warm-bloodedanimal subjects, each unit containing a predetermined quantity of activematerial calculated to produce the desired therapeutic effect inassociation with the required pharmaceutical diluent, carrier orvehicle. The specifications for the novel single dosage forms of thisinvention are dictated by and are directly dependent on (a) the uniquecharacteristics of the active material and the particular therapeuticeffect to be achieved, and (b) the limitations inherent in the art ofcompounding such an active material for therapeutic use in warm-bloodedanimals as disclosed in detail in this specification. Examples ofsuitable oral single dosage forms in accord with this invention aretablets, capsules, pills, powder packets, granules, wafers, cachets,teaspoonfuls, dropperfuls, ampules, vials, segregated multiples of anyof the foregoing, and other forms as herein described.

The following examples illustrate the invention but are not to beconstrued as limiting.

REPRESENTATIVE PREPARATIONS OF SEED CRYSTALS

Since crystal formation is facilitated by seeding, it has been foundadvantageous to first prepare seed crystals. The procedures employedhave been either to carry out a small preparation using about 10 gramsof POE ester and producing the crystals via spontaneous crystallizationor to effect a solvent of crystallization exchange.

Illustrative of a small scale of preparation is that of phosphoric acidethanolate salt from POE ester: 10 grams of methyldopa (R,S)-POE ester,3.4 grams of 85 percent phosphoric acid are added to a mixture of 70milliliters of toluene and 35 milliliters of ethanol at roomtemperature, then allowed to stand at 5°-10° C. overnight to obtaincrystals of (R,S)-1-pivaloyloxyethyl(S)-3-(3,4-dihydroxyphenyl)-2-methylalaninate, phosphoric acidmonoethanolate salt product.

Illustrative of small scale preparation of phosphoric acid ethanolatesalt via solvent of crystallization exchange: 10 grams of (R,S)-POEester phosphoric acid salt monohydrate from an earlier synthesis isdissolved in 200 milliliters of hot anhydrous ethanol and the mixtureallowed to cool to room temperature whereupon crystal formation starts.The mixture is allowed to stand overnight to complete the crystalformation and separation of (R,S)-1-pivaloyloxyethyl(S)-3-(3,4-dihydroxyphenyl)-2-methylalaninate, phosphoric acidmonoethanolate salt product.

EXAMPLE 1 (R,S)-1-Pivaloyloxyethyl(S)-3-(3,4-Dihydroxyphenyl)-2-methylalaninate, Phosphoric AcidMonoethanolate Salts A. Preparation of the POE Ester Base

Under an atmosphere of nitrogen, 425 grams (2.0 moles) of(S)-3-(3,4-dihydroxyphenyl)-2-methylalanine (methyldopa), 352milliliters (2.0 moles) of 1-chloroethyl pivalate, 142 grams of 4Amolecular sieve (1/16" pellets), and 850 milliliters of tetramethylureawere mixed together, heated to 75° C., and thereafter maintained attemperatures in the range 70°-75° C. for 30 to 36 hours. During thisperiod, tne methyldopa dissolved with the formation of(R,S)-1-pivaloyloxyethyl ester of methyldopa. Thereafter, the reactionmixture containing ester was cooled to 25° C. and diluted with 3500milliliters of toluene. The organic molecular sieve containing layer wasthen washed with three 4.5 liter portions of saturated sodiumbicarbonate solution at 15°-20° C. and filtered to clarify the solutionto recover organic solution containing POE ester of methyldopa.

B. Preparation of the Salt Product

To the organic solution there was added about 3 liters of ethanol,followed by 168 grams (1.46 moles) of 85 percent phosphoric acid and 4grams of methyldopa R,S,-POE ester phosphoric acid monoethanolate seed,and the mixture aged by stirring at about 20°-25° C. for about 20 hoursto obtain an R,S-1-pivaloyloxyethylS-3-(3,4-dihydroxy-phenyl)-2-methylalaninate, phosphoric acid salt,monoethanolate product. The product was washed twice with 400milliliters of ethanol-toluene and dried in vacuo to obtain 375.4 grams(38.8 percent yield) of a purified crystalline product. Analyticalresults were as follows:

KF (Karl Fischer) =0.50 percent (H₂ O)

GLC=8.67 percent EtOH (9.53 percent theory for 1 mole EtOH)

Titration HClO₄ =99.9 (corr. for EtOH and H₂ O); NaOH=100.0 (corr. forEtOH and H₂ O)

LC=97.6 percent (weight percent; corr. for EtOH and H₂ O) [α]₃₆₅ ²⁵=-18.2° (38.6 percent S-isomer; 61.4 percent R-isomer, based on rotationof -41.7° for pure S-isomer and +14.3° for pure R-isomer).

EXAMPLE 2 (R,S)-1-Pivaloyloxyethyl(S)-3-(3,4-Dihydroxyphenyl)-2-methylalaninate, Phosphoric AcidMonoethanolate Salts

Approximately 200 grams (0.59 mole) of methyldopa, R,S-POE ester (97percent purity) followed by 68 grams (0.59 mole) of 85 percentphosphoric acid were charged to a mixture of 1,400 milliliters oftoluene and 700 milliliters of ethanol at 21° C. to obtain a light ambersolution of the mixture. The solution was then seeded with a previouslyprepared POE ester salt whereupon crystal formation occurred withprecipitation of POE ester salt. The crystalline slurry was then allowedto stand (aged) at 5°-10° C. for about 16 hours to allow crystallizationof the isomeric POE ester salt; thereafter, the mixture was filtered andwashed with 650 milliliters of toluene: ethanol (2:1) followed by 1200milliliters of hexane and dried under a nitrogen atmosphere, in vacuo at25° to obtain 267 grams (93 percent yield) of methyldopa R,S-POE esterphosphate ethanolate.

Analytical results were as follows:

KF 0.59 percent

GLC=8.52 percent EtOH

Titration HClO₄ =100.4 percent

NaOH=99.3 percent

L.C. 98.2 percent (weight percent; corr. for EtOH and H₂ O)

Elemental Analysis: Calculated for C₁₇ H₂₅ NO₆ .H₃ PO₄.C₂ H₅ OH: C,47.20; H, 7.10; N, 2.90; P, 6.41 (Corr. for EtOH and H₂ O).

Found C, 46.62; H=7.19; N, 2.88; P, 6.34. [α]₃₆₅ ²⁵ =-12.6° (about 52percent (S) isomer).

EXAMPLE 3 (R,S)-1-Pivaloyloxyethyl(S)-3-(3,4-Dihydroxyphenyl)-2-methylalaninate Phosphoric AcidMonohydrate Salts

200 grams (0.589 mole, 96.4 percent purity) of (R,S)-POE ester basemixture in 4.9 liters of ethyl acetate was stirred under nitrogen at20°-25° C. for about twenty minutes and the mixture then filtered toremove insoluble material which was primarily methyldopa. The latter waswashed with two 250 milliliter portions of ethyl acetate and the washadded to the solution containing the base mixture which then amounted to5.45 liters. To this solution was added dropwise over about a 15 minuteperiod, a solution of 39.2 milliliters (0.571 mole corrected for 3percent methyldopa impurity) of 85 percent phosphoric acid in 58.8milliliters of water. The addition funnel was rinsed with 430milliliters of ethyl acetate and the combined ethyl acetate solution(5.98 liters) was seeded with a previously prepared (R,S)-POE esterphosphate salt whereupon crystallization started. The crystallizingmixture was stirred at room temperature for 2 hours, then cooled to 0°C. over a 2 hour period. The crystals which formed at room temperaturewere primarily those of the S-isomer salt as determined by sampling thecrystals and checking the TLC (thin layer chromatography) of the solid(system employed: silica gel plates 60:25:10:5-EtOAc:n-BuOH:HCOOH:H₂ O;I₂ visualization, R_(f) -S isomer=0.6; R_(f) - R isomer=0.7). Thereafterthe mixture was further cooled to a temperature of -10° C. and allowedto stand at this temperature to crystallize the remaining ester salt.

During this period, the progress of the crystallization was followed byTLC and rotation. After about 19 hours total reaction time, the whitecrystalline (R,S)-1-pivaloyloxyethyl(S)-3-(3,4-dihydroxyphenyl)-2-methylalaninate phosphoric acidmonohydrate salt was recovered by filtration. (A total solidsdetermination of the filtrate indicated that about 10 percent of theproduct was still in the filtrate.) The white crystalline salt waswashed with three 490 milliliter portions of ethyl acetate and dried toa constant weight of 231 grams (corrected for 2 grams of seed) whichamounted to a yield of 86 percent (89.3 percent when corrected for thepurity of the starting (R,S)-POE ester base). The salt had a meltingpoint of 117.5°-119.5° (dec.).

Results of analyses of the product were as follows:

KF =4.78 percent (theory 3.95 percent)

Titration (HClO₄) =99.3 percent uncorr.

UV A% 280 nm =62.6

LC =73 weight percent compared to standard POE base; (98.7 weightpercent corrected for KF)

X-ray - crystalline

[α]₃₆₅ ²⁵ nm=-11.6° 53.9 percent S-isomer; 46.1 percent R-isomer

Elemental analyses: Calculated for C₁₇ H₂₅ NO₆.H₃ PO₄.H₂ O (455.5). C,44.84; H, 6.64; N, 3.08; P, 6.80. Found: C, 44.55; H, 6.56; N, 2.83; P,6.58.

EXAMPLE 4 (R,S)-1-Pivaloyloxyethyl(S)-3-(3,4-Dihydroxyphenyl)-2-methylalaninate (+)-TartaricAcid/Phosphoric Acid (3/1) Monohydrate Salts

33.9 grams (0.1 mole, 99.5 percent purity) of (R,S)-POE ester base in800 milliliters of ethyl acetate was stirred at room temperature for 15minutes and the mixture filtered to remove impurities. The filteredsolid was washed with two 15 milliliter portions of ethyl acetate andthe ethyl acetate solutions combined. To the combined solution there wasadded 1 gram of a previously prepared (R,S)-POE-(+)-tartaric acid saltas seed, followed by a slow addition of 11.25 grams (0.075 mole) of(+)-tartaric acid in 10 milliliters of water. The addition funnel wasrinsed with 3 milliliters of water into the mixture and the mixture thenstirred at ambient temperature for about 2 hours whereupon crystalscomprising primarily of the salts of the S-isomer separated in thereaction mixture. To the resulting mixture, an additional gram of(R,S)-POE ester (+)-tartaric acid salt in 70 milliliters of ethylacetate was added as seed and the mixture cooled by a circulatingethylene glycol bath whereupon crystallization of the R-isomer salt tookplace. After aging overnight at about -12° C. substantially equalamounts of the R- and S-isomer was found to have formed. (An estimate ofthe ratio was made by thin layer chromatography employing 60:25:10:5EtOAc:n-BuOH:HCOOH:H₂ O; system on silica gel plates with I₂visualization. The Rf for S-isomer was 0.6 and R-isomer, 0.7).

To the crystallization mixture containing R and S tartrate salts of POEester base, there was added a solution of 1.7 milliliters of 85 percentphosphoric acid in 3 milliliters of water and 100 milliliters of ethylacetate containing 0.5 gram of (R,S)-POE ester phosphate salt seeds, andstirring continued at -12° C. for additional 16-20 hours whereupon R andS phosphate salts of POE ester base crystallized. The mixed crystalswere then recovered by filtration and washed with two 25 milliliterportions of cold ethyl acetate and dried at 25° C./1 mm to obtain(R,S)-1-pivaloyloxyethyl (S)-3-(3,4-dihydroxyphenyl)-2-methylalaninate(+)-tartaric acid/phosphoric acid (3:1) monohydrate salt in a yield of44 grams (90 percent of theoretical). Tne product had a meltingtemperature of 122.5°∝128.5° (dec). Elemental analysis for phosphorusand other analytical information are as follows:

Elemental for P: Calculated P, 1.70. Found P, 1.74

KF 3.0 percent

UV 280 nm A%=56.4

Titration (HClO₄ and NaOH)

72.4 percent POE Tartrate salts (uncorr.);

24.2 percent POE Phosphate salts (uncorr.);

99.5 percent POE Ester (corrected for KF)

Equivalent weight=492.3 (HClO₄)

LC 67.3 weight percent as (R,S)-POE ester base; 97.6 percent pure astartrate and phosphate salts hydrate; 0.3 percent α-methyldopa

Microscopy: Crystalline material, <10μ with agglomerates; exhibitsbirefringence.

EXAMPLE 5 (R,S)-1-Pivaloyloxyethyl(S)-3-(3,4-Dihydroxyphenyl)-2-methylalaninate (+)-Tartaric AcidMonohydrate Salts

33.9 grams (0.1 mole, 98 percent purity) of (R,S)-POE ester base in 625milliliters of ethyl acetate was stirred under nitrogen at 20°-25° C.for about 20 minutes then filtered to remove insoluble impurities. Thefiltrate was washed with two 50 milliliter portions of ethyl acetate,the ethyl acetate solutions combined and then diluted with additional250 milliliters of ethyl acetate. To the resulting solution while undernitrogen atmosphere was added a solution of 15 grams (0.1 mole) of(+)-tartaric acid in 25 milliliters of water and the solution seededwith 2.4 grams of previously prepared (R,S)-POE ester (+)-tartaric acidsalt mixture whereupon crystallization began. Stirring was continued forabout 2 hours at room temperature with separation predominantly of thesalt of the S-isomer.

At the end of this period, the mixture was diluted with 1000 millilitersof ethyl acetate, cooled to 0° C. over a 2 hour period, seeded withadditional 2.4 grams of (R,S)-POE ester (+)-tartaric acid salt mixtureand maintained at 0° C. for about 4 hours to obtain white, crystalline(R,S)-1-pivaloyloxyethyl (S)-3-(3,4-dihydroxyphenyl)-2-methylalaninate(+)-tartaric acid monohydrate salts in a yield of 42.0 grams (aftercorrection for seed crystals) or 86 percent of theoretical.

Elemental analyses and rotation properties of the product were asfollows:

Calculated for: C₁₇ H₂₅ NO₆.C₄ H₆ O₆.H₂ O (507.5): Calcd.: C, 49.70; H,6.55; N, 2.76. Found: C, 49.35; H, 6.34; N, 2.59.

Rotation: [α]₃₆₅ ²⁵ =+8.9° (C=5, H₂ O)*

EXAMPLE 6

(R,S)-1-Pivaloyloxyethyl (S)-3-(3,4-Dihydroxyphenyl)-2-methylalaninate,Phosphoric Acid, Monoisopropanolate Salts

In an operation carried out in a manner similar to that described inExample 1, 42.3 grams (0.2 mole) of methyldopa, 40.3 milliliters (0.24mole) of 1-chloroethyl pivalate, 14 grams of 4A molecular sieve pellets(1/16") and 85 milliliters of tetramethylurea are mixed together andheated in an atmosphere of nitrogen to 70° C. and maintained at thistemperature for about 30 hours. During this time, the methyldopadissolves with the formation of (R,S)-1-pivaloyloxyethyl ester product.Thereafter, the reaction mixture is cooled to 25° C. and diluted with380 milliliters of toluene. The mixture is then washed with three 500milliliter portions of saturated sodium bicarbonate solution andfiltered to clarify the solution. To this solution is added 300milliliters of isopropanol followed by 16.6 grams (0.14 mole) of 85percent phosphoric acid and 0.100 gram of methyldopa (R,S)-POE esterphosphoric acid monoisopropanolate seed whereupon the productcrystallizes. The crystal-containing slurry is aged for 16 hours at20°-25° C., then filtered and washed with two 75 milliliter portions ofisopropanol and dried in vacuo to obtain (R,S)-1-pivaloyloxyethyl(S)-3-(3,4-dihydroxyphenyl)-2-methylalaninate, phosphoric acid,monoisopropanolate salt.

EXAMPLE 7 (R,S)-1-Pivaloyloxyethyl(S)-3-(3,4-Dihydroxyphenyl)-2-methylalaninate, Phosphoric Acid,Monoisopropanolate Salts

10 grams (0.03, 96 percent purity) of (R,S)-POE ester base was added toa mixture of 100 milliliters of isopropanol, 3 grams of 85 percentphosphoric acid and 3 milliliters of water. The mixture slurry wasseeded with a small amount of previously prepared POE ester, phosphoricacid isopropanolate salt whereupon immediate heavy precipitation of(R,S)-1-pivaloyloxyethyl (S)-3-(3,4-dihydroxyphenyl)-2-methylalaninate,phosphoric acid, isopropanolate salt product precipitated as crystallinewhite powder. Optical rotation of the mixture was [α]₃₆₅ ²⁵ =-11.3°(C=1.0, H₂ O), indicating about 54 percent of S-isomer and 46 percent ofthe R isomer.

EXAMPLE 8 (R,S)-1-Pivaloyloxyethyl(S)-3-(3,4-Dihydroxyphenyl)-2-methylalaninate, Maleic Acid,Monoethanolate Salts

In a manner similar to that described in Example 1 and 6, a mixture of21.2 grams (0.10 mole) of methyldopa, 20.15 milliliters (0.12 mole) of1-chloroethyl pivalate, 7 grams of 4A molecular sieve (1/16" pellets)and 43 milliliters of tetramethylurea are heated to 70° C. andmaintained at this temperature for 30 hours under an atmosphere ofnitrogen. During this period a reaction takes place with dissolution ofmethyldopa and formation of the R,S-POE ester of methyldopa. The mixtureis allowed to cool to 25° C. and then diluted with 205 milliliters oftoluene. It is then washed with three 250 milliliter portions ofsaturated aqueous sodium bicarbonate solution. The organic layer afterseparation from the aqueous layer is clarified by filtration and dilutedwith 125 milliliters of ethanol. A solution of 8.12 grams (0.7 mole) ofmaleic acid in 20 milliliters of ethanol and 0.1 gram of a previouslyprepared maleic acid, monoethanolate salt as seed is added whereuponmaleic acid monoethanolate salt of R,S-POE ester of methyldopa begins toprecipitate as crystals. The mixture is aged for about 16 hours at20°-25° C. to complete the formation of crystals and then filtered torecover the solid product. The product is washed with 250 milliliterportions of ethanol and dried in vacuo to obtain purified(R,S)-1-pivaloyloxyethyl (S)-3-(3,4-dihydroxyphenyl)-2-methylalaninate,maleic acid, monoethanolate salt.

EXAMPLE 9 (R,S)-1-Pivaloyloxyethyl(S)-3-(3,4-dihydroxyphenyl)-2-methylalaninate, Phosphoric Acid,Monomethanolate Salts

In a similar operation similar to that described in Example 2 under anatmosphere of nitrogen, 200 grams of methyldopa, R,S-POE ester (of 97percent purity) and 68 grams of 1400 milliliters of toluene and 700milliliters of methanol at 21° C. to obtain a light amber coloredorganic solution. The solution is seeded with 0.5 gram of a previouslyprepared POE ester phosphoric acid methanolate salt whereupon crystalformation starts with the precipitation of the salt product crystals inthe reaction mixture. The mixture is allowed to stand for about 16 hoursat temperature range of 20°-25° C. to complete the formation of thecrystals of(R,S)-1-pivaloyloxyethyl(S)-3-(3,4-dihydroxyphenyl)-2-methylalaninate,phosphoric acid, monomethanolate salt product. The product is washedwith two 500 milliliter portions of methanol and dried in vacuo toobtain a purified product.

EXAMPLE 10 (R,S)-1-Pivaloyloxyethyl(S)-3-(3,4-dihydroxyphenyl)-2-methylalaninate, Sulfuric Acid,Monoethanolate Salts

In a similar manner, under a nitrogen atmosphere, a mixture of 200 gramsof methyldopa, R,S-POE ester (of 97 percent purity) and 31 grams ofconcentrated sulfuric acid are added portionwise to a mixture of 1400milliliters of toluene and 700 milliliters of ethanol at about 20° C.The resulting mixture is seeded with 0.5 gram of crystalline POE estersulfuric acid ethanolate salt whereupon crystals start to separateimmediately in the reaction mixture. The crystalline slurry is allowedto stand in the temperature range of 20°-25° C. for 16 hours andthereafter filtered, washed with two 600 milliliter portions of ethanoland dried in vacuo to produce(R,S)-1-pivaloyloxyethyl(S)-3-(3,4-dihydroxyphenyl)-2-methylalaninate,sulfuric acid, monoethanolate salt product.

EXAMPLE 11 (R,S)-1-Pivaloyloxyethyl(S)-3-(3,4-Dihydroxyphenyl)-2-methylalaninate, Phosphoric Acid,Monoisopropanolate Salts

To a slurry of 10.2 grams of methyldopa, R,S-POE ester in 100milliliters of isopropanol was added 3.5 grams of 85 percent phosphoricacid, followed by 1 milliliter of water which produced a solution. Themixture was stirred for 4 hours at 25° C. whereupon a heavy crystallineprecipitate formed. The mixture was maintained at 0°-5° C. overnight andthen filtered and washed with two 50 milliliter portions of isopropanolto obtain 13.2 grams of (R,S)-1-pivaloyloxyethyl(S)-3-(3,4-dihydroxyphenyl)-2-methylalaninate, phosphoric acid,monoisopropanolate salt product as a white solid.

Analytical results were as follows:

KF=1.62 percent

Titration (HClO₄)=93.5% uncorr.

[α]₃₆₅ ²⁵ nm=-14.0°

X-ray =crystalline/none crystalline

Elemental analyses: Calc'd. for C₁₇ H₂₅ NO₆.H₃ PO₄.H₂ O (m.w. 455); C,44.84; H, 6.64; N, 3.08; P, 6.80. Found: C, 46.79; H, 7.29; N, 2.80; P,4.50.

EXAMPLE 12

In an operation similar to that described in Examples 7 and 11,(R,S)-1-pivaloyloxyethyl (S)-3-(3,4-dihydroxyphenyl)-2-methylalaninate,phosphoric acid, monoethanolate salts are prepared by mixing together 10grams of R,S-POE ester base, 3 grams of 85 percent phosphoric acid and100 milliliters of ethanol at ambient temperature, then maintainingovernight at 0°-5° C. and recovering by filtration.

PREPARATION OF STARTING MATERIAL

The (R,S)-1-pivaloyloxyethyl(S)-3-(3,4-dihydroxyphenyl)-2-methylalaninate starting material is knownin the art and also may be prepared by intimately mixing with warming(S)-2-(3,4-dihydroxyphenyl)-2-methylalanine with 1-chloroethyl pivalatein an aprotic solvent such as dimethylsulfoxide or tetramethylurea inthe absence of an acid binding agent according to the method more fullydescribed in U.S. Pat. No. 3,988,341 or by the hydrogenation methoddescribed in the aforecited J. Med. Chem. 21, 746 (1978). It also may beprepared following the procedure of Example 1A.

What is claimed is:
 1. A process for preparing in substantiallycrystalline form, a diastereomeric salt mixture of 1-pivaloyloxy-ethylester of (S)-3-(3,4-dihydroxyphenyl)-2-methylalanine with an inorganisacid and obtained as an alkanolate which comprises mixing together toform a slurry and causing to react purified (R,S)-1-pivaloyloxyethyl(S)-3-(3,4-dihydroxyphenyl)-2-methylalaninate, the appropriate inorganicacid for the desired salt and lower alkanol in the absence of awater-immiscible solvent.
 2. A process according to claim 1 in which theacid is phosphoric acid and the alkanol is ethanol.
 3. A processaccording to claim 1 in which the acid is phosphoric acid and thealkanol is isopropanol.